Method of correcting the rotating speed of vehicle wheels sensed by wheel sensors

ABSTRACT

Vehicle speed is measured during slip-free travel of the wheels and compared with the wheel speeds to determine correction values which correct for different wheel diameters. The correction values are used to continuously correct the wheel speeds.

BACKGROUND OF THE INVENTION

It is known to measure the speed of vehicle wheels by means of sensorsand to use the measured speeds to control the wheel slippage. Wheelslippage can be caused by overbraking of wheels (brake slippage), toogreat a drive torque (drive slippage) or by the drag torque of theengine when the friction coefficient between road and tire is too small.In an ABS, the brake slippage is controlled by changing the brakepressure In a drive slip control (ASR), the drive slippage is controlledby changing the drive torque and/or the brake. In an engine drag torquecontrol, the slippage is controlled by changing the drive.

The tires of a vehicle may have different diameters and thus differentwheel speeds can be measured at the individual wheels. This can lead toan inaccuracies in the above mentioned control procedures.

From European patent A2 0133381 to which U.S. Pat. No. 4,566,737corresponds, it is know to detect during travel different tire diametersby difference formation of wheel speed signals and to correct themeasured speed values of wheels with different tire diameters. Forcontrolling purposes, the resulting values can be set into relation toone another.

SUMMARY OF THE INVENTION

Determining the vehicle speed in accordance with the invention ensures avery good approach to the situation.

The detection of diameter differences of the wheels and the correctingcalculations are preferably carried out when there is no braking, whennone of the control procedures is being carried out, when not drivingthrough a curve (small steering angle signal or small transverseacceleration or approximately the same rotating speed at the wheels andthe axles), when the vehicle acceleration or deceleration is small, whenthe wheels are not at all or only slightly accelerated or deceleratedand/or when only a small engine torque is coupled to the driven wheels.The latter can be signaled by a small engine output torque or noconnection between engine and driven wheels or, in case of an automatictransmission, when setting the lever to the "N-position." In ABScontrols, the zero torque determination is not possible by means of DKV-and n_(mot) -scan since this is ASR-specific information.

Instead, in case of ABS control, it is possible to read in the line tothe fuel consumption display (KVA) of an already present injectionsystem (Motronic, Jetronic, or the like). The values are approximatelyproportional to the injection time T_(i). Knowing the correspondingengine characteristics, it is possible to determine the engine torquefrom T_(i) or KVA. The above criteria can be used in differentcombinations in order to determine the slippage-free wheel run. It isalso possible to carry out the measuring and correcting merely in anaverage speed range.

The change of the dynamic wheel diameter is a non-linear function of thevehicle speed. In order to carry out a non-linear correction, acorrection by means of speed-dependent correction values over the entirespeed range is possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram for the working of the method in accordancewith the invention,

FIG. 2 is a flow diagram for the working of the method,

FIGS. 3 and 4 are an explanatory table and diagram, respectively,

FIG. 5 is a block diagram of another embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the FIG. 1, the speed sensors 1-4 associated with the four wheels ofa vehicle supply wheel speeds V₁ to V₄ to a block 5. A further block 6activates block 5, if, as it is assumed here, there is no braking (BLS),ABS and ASR are not in operation (ABS and ASR), the vehicle accelerationor deceleration is smaller by a prescribed value a₁, the transverseacceleration a_(Q) is smaller than a value a₂ and if the vehicle movesat a speed between 20 and 110 km/h (12 and 70 mph).

If these conditions are met, a slippage free travel is assumed. It isassumed that the criterion a_(Q) is smaller than a₂ and that it is addedonly later, prompted by a supplementary signal

In block 5 the following differences are continuously formed:

    ΔV.sub.1 =/(V.sub.1 -V.sub.2)/

    ΔV.sub.2 =/(V.sub.1 -V.sub.3)/

    ΔV.sub.3 =/(V.sub.1 -V.sub.4)/

    ΔV.sub.4 =/(V.sub.2 -V.sub.3)/

    ΔV.sub.5 =/(V.sub.2 -V.sub.4)/

    ΔV.sub.6 =/(V.sub.3 -V.sub.4)/

and the smallest difference ΔV is determined. From the speeds, forexample, V₂ and V₃ of this difference ΔV min=ΔV₄, the mean value##EQU1## is formed in a block 16.

A mean value ΔV of the difference is preferably determined via amultiple of successive measurements in a block 5. The mean value ispreferably a weighted value which is determined according to thefollowing equation: ##EQU2##

In this equation the mean value ΔV (t-1) which was obtained as the lastvalue (time t-1) from the mean value formation (m-constant and e.g.1000) is provided with the factor m and the newly (time t) determineddifferential value ΔV (t) is added thereto and the sum is divided by(m+1). At the start of the computation, ΔV is zero.

According to the smallest difference ΔVmin resulting therefrom, thosewheel speeds are selected which have the smallest difference from oneanother and the above said mean value V is formed in a block 16.

The comparator 7 checks whether the difference of the two selectedwheels is smaller than a prescribed value, for example smaller than 1%.If this is the case, a block 8 is activated. Otherwise, the measurementis started again by deactivating block 5.

If the difference is smaller than for example 1% correction factors##EQU3## (according to the above example) are formed where those wheelspeeds are set into relation to V, which are not included in V. This issignaled from block 5 via line 5a to block 8. From the successivelyobtained correction values K₁ and K₄, the weighted mean values K₁ and K₄are formed in block 8 according to the equations ##EQU4## where m is aconstant number, for example 1000,

K_(i) =1 at the beginning of the computation.

The K values can be stored in a further block 9 and be checked forplausibility. It is assumed that K_(i) must not exceed a prescribedvalue K_(limits), which, for example, is prescribed by the diameter ofthe emergency wheel.

The values K₁ and K₄ which are determined in the assumed example aresupplied by lines 9' to multipliers 10 and 13 where the corrected speeds

    V.sub.1K =K.sub.1 V.sub.1

    V.sub.4K =K.sub.4 V.sub.4

are formed from the measured speeds V₁ and V₄. Via terminals 14-17, thepartly corrected and partly not corrected speed signals V_(1K), V₂, V₃,V_(4K) are available for further evaluation.

Once the correction values were determined, it is possible to furtherspecify the criteria for correction determination and, in addition, toadd the straightforward directed travel as another condition to thedetermination. For this purpose, block 6 is switched via an OR-gate 15when the correction signal K_(i) is formed such that it activates block5 only when straightforward directed travel is added, i.e. when a_(Q)<a₂.

The full diagram of FIG. 2 now serves to explain a slightly differentmethod. After the start 20, the smallest difference ΔVmin is determinedin 21 and the mean value V is formed. 22 checks whether the determinedvalues are plausible (e.g. ΔVmin/V<1%). If this is not the case, thecorrection values K are calculated in 23 and in 24 checked forplausibility ##EQU5##

In this case, the wheel speeds are also corrected in 25. If, however, itis determined in blocks 22 or 24 that the values supplied are notplausible, the last preceding plausible correction value is used via 26for correction.

During slippage-free travel, the wheel-specific correction values K_(i)can be determined according to the relation ##EQU6## where V_(i) is thewheel speed of the i-th wheel and K_(i) is the correction factorthereof. The values for K_(i) are then stored and the measured wheelspeed is continuously corrected with the stored correction value K_(i)(V_(i) corr =K_(i) V_(i)).

The use of the correction in accordance with the invention does notcause problems in the control system in case the tire size is changed orwhen snow chains, an emergency wheel or the like are used.

It is possible to specify the conditions for the determination of thevehicle speed (e.g. the limits for acceleration, for the steering angleor the transverse acceleration or the difference of the wheel rotationsof one axis or for the small engine torque) in dependency upon the timethat has elapsed since the start or in dependency upon the number of thecorrection cycles that have occurred since the start. This is shown inthe table of FIG. 3. Passing a curve is defined by the speed differenceΔV of the steered wheels and the pedal position defines the engine zerotorque by the zero torque value.

Since it is possible that the deviation of one wheels is so big that theASR is in operation from the start, it may be possible that there is nocorrection factor determination. Therefore, the system determines at thebeginning after 20 sec a correction factor despite ASR. The waiting timeincreases with an increasing time T beginning with the start. After 20minutes, for example, it is possible to completely omit thisdetermination. The use of time T hence makes the determination of thevehicle speed more responsive.

A stored correction value can be replaced by a new one only if thedeviation exceeds a deviation limit. This value can also be made todepend upon the time after the start such that it decreases with thetime passing by. This is shown in FIG. 4 with the two limits 40 and 41.The value is stored when the newly determined value is above or belowthis limit. There is also another time limit included which isreferenced as 42. Correction values are not replaced before 1 minute haselapsed.

In FIG. 5, the wheel speed are supplied via terminals 61 to a block 60for the determination of new correction values. Block 60 determines thecorrection values only if it is activated by a block 62. This is doneonly if it recognizes slippage-free conditions from the availablesignals. The conditions for determining that there is no slippage can bemade to depend upon the time that has elapsed since the start (cf. FIG.3). Therefore, a time element 63 which is set at the start supplies atime signal to block 62.

If new correction factors are determined in a block 60, they aresupplied to a storing and comparing element 64. Each old stored value isreplaced by the new respective value if this new value deviates from theold value by at least one limit preset by a block 65. In accordance withFIG. 4, this limit is subject to variation after the start. (line 66). Alimit signal and, hence, a correcting comparative value is inhibited byan AND-gate 67 as long as a minimum time has not elapsed since the startduring which the time element 68 has not supplied a signal.

In multipliers 69, the continuously determined wheel speeds aremultiplied with the correction factors which are ultimately stored inelement 64. At a terminal 70, the corrected wheel speeds are availablefor evaluation in an ABS or ASR.

We claim:
 1. Method for correcting the rotating speed of vehicle wheels,said method comprising the following steps:measuring the rotating speedof each vehicle wheel, determining when there is no slippage of saidvehicle wheels, determining the difference in speed between each twovehicle wheels when there is no slippage of said vehicle wheels,determining the minimum said difference in speed, determining theaverage speed of the two wheels which have said minimum difference inspeed, wheels other than said two wheels which have said difference inspeed being remaining wheels, determining correction values for eachsaid remaining wheel based on the speed of each said remaining wheel andsaid average speed, and correcting the measured speed of each saidremaining wheel based on respective said correction values.
 2. Method asin claim 1 wherein said determining of no slippage is made only whenthere is no slippage control in the operation, said slippage controlcomprising at least one of ABS, ASR, and engine drag torque control. 3.Method as in claim 2 wherein said differences are determined only afteran elapsed time T of vehicle movement.
 4. Method as in claim 3 wherein,when slippage control is detected, the correction value is determinedwhen a waiting time has elapsed, and no further correction values aredetermined after said waiting time has elapsed.
 5. Method as in claim 4wherein said waiting time is increased in dependency upon time T. 6.Method as in claim 3 wherein said average speed is determined only whenengine torque is below a prescribed value, said prescribed valuedecreasing with elapsed time T.
 7. Method as in claim 3 wherein furthercomprising determining a vehicle acceleration, said differences beingdetermined only when said acceleration is within prescribed limits, saidprescribed limits decreasing with elapsed time T.
 8. Method as in claim3 further comprising determining a transverse vehicle acceleration anddetermining said differences only when said transverse acceleration isless than a prescribed value, said prescribed value decreasing withelapsed time T.
 9. Method as in claim 1 wherein said average speed isdetermined only when there is no braking.
 10. Method as in claim 1wherein said average speed is determined only when engine torque isbelow a prescribed value.
 11. Method as in claim 1 wherein saiddifferences are determined only when said average speed is within aprescribed range.
 12. Method as in claim 1 further comprisingdetermining a vehicle acceleration, said difference being determinedonly when said acceleration is within prescribed limits.
 13. Method asin claim 1 further comprising determining transverse vehicleacceleration and determining said differences only when said transversevehicle acceleration is less than a prescribed value.
 14. Method as inclaim 1 wherein said wheel speed differences are determined atsuccessive times for each two wheels, said minimum difference beingdetermined by taking the mean of successively determined differences foreach two wheels and selecting the lowest mean.
 15. Method as in claim 14wherein said means is a weighted means ΔV determined at time t accordingto ##EQU7## where m is a constant, ΔV(t-1) is the mean determined attime (t-1), and ΔV(t) is the difference determined at successive time t.16. Method as in claim 1 wherein said correction values are determinedonly when said wheels having said minimum difference in speed havespeeds which deviate from each other by less than a prescribed value.17. Method as in claim 1 wherein the measured speeds V_(i) of theremaining wheels are corrected according to V_(Ki) =K₁ V_(i) where K_(i)is the correction value for the remaining wheel and V_(Ki) is thecorrected speed of the remaining wheel.
 18. Meethod as in claim 17wherein said correction values are determined according to K₁ =V/V_(i)for each remaining wheel i, where K_(i) is the correction value for theremaining wheel i, V is the average speed of the two wheels which havethe minimum difference, and V_(i) is the speed of the remaining wheel i.19. Method as in claim 17 wherein said correction value K is a weightedcorrection value K_(i) determined at time t according to ##EQU8## wherem is a constant, K_(i) (-1) is the correction value determined at time(t-1), and K_(i) (t) is the correction value determined at successivetime t.
 20. Method as in claim 19 further comprising determining thedeviation of K_(i) (t) from K_(i) (t-1) and limiting said deviation to aprescribed value.
 21. Method as in claim 17 further comprisingdetermining the deviation of K_(i) from 1 and limiting said deviation toa prescribed value.
 22. Method as in claim 12 wherein said prescribedvalue decreases with time.
 23. Method as in claim 1 wherein saidcorrection values are determined according to K_(i) =V/V_(i), whereK_(i) is the correction value for the remaining wheel i, V is theaverage speed of the two wheels which have the minimum difference, andV_(i) is the speed of the remaining wheel i.
 24. Method as in claim 1further comprisingstoring said correction values for each said remainingwheel, newly determining correction values after said correction valuesare stored, comparing newly determined correction values to respectivestored correction values, and replacing said stored correction valuesonly with said new determined correction values only when said newlydetermined correction values exceed respective said stored correctionvalues by respective prescribed limit values.
 25. Method as in claim 24wherein in said prescribed limit values decrease with elapsed time T ofvehicle movement.
 26. Method as in claim 24 wherein said storedcorrection values are replaced only after a minimum elapsed time aftervehicle departure.